To estimate the potential of a machine to perform work and the consumption of energy, the concept of power was introduced. Power is work per unit time, or the time rate of doing work.

There are many different units of power. In the cgs system, the unit is the erg per second (erg/s), One erg per second, however, is very little power and, hence, not useful in practical life. A unit in much wider use is the joule per second, or watt (W): \(1\,\mathrm{W} = 1~\mathrm{J/s} = 1\times 10^{7} \textrm{erg/s}\). When this unit is not enough, it is multiplied by a thousand. The new unit is called a kilowatt: (kW).

From the early days of technology we have inherited the unit of power called horsepower (hp). This name had a special meaning at that time. A person buying a 10-horsepower machine would conclude that it took the place of 10 horses, even if he knew nothing of units of power.

Naturally, there are no two horses alike. The first person to introduce this unit of power apparently thought that the average horse can do \(75\,\mathrm{kgf\cdot m}\) of work per second. Thus, one horsepower was arbitrarily defined as \(75\,\mathrm{kgf\cdot m/s}\). A heavy draught-horse can work at a rate greater than 1 hp; especially when starting. But the power of an average horse is about 0.5 hp. The relation of horsepower to kilowatt is \(1~\textrm{hp} = 0.735\,\mathrm{kW}\).

In everyday life and in technology we deal with a great variety of machines. The motor of the turntable of a record player has a power output of about \(10\,\mathrm{W}\), the engine of the Soviet car Volga 100 hp or \(73\,\mathrm{kW}\), and the engines of the Soviet passenger airliner “IL-18” \(16 000\) hp. A small electric power station used to supply a cooperative farm with electricity has a power output of about \(100\,\mathrm{kW}\), whereas the Krasnoyarsk hydroelectric plant on the Yenisei river in Siberia has a record power output of 5 million kilowatts.

The units of power we have elaborated on give us a clue to a unit of work or energy used exclusively for electricity, namely the kilowatt-hour (kW ·h). A kilowatt-hour is the work produced by a source with the power output of one kilowatt in the course of one hour. From this new unit it is easy to transfer to the old ones: \begin{align} 1\,\mathrm{kW} & = 3.6\times 10^6\,\mathrm{J}\\ & = 861\,\mathrm{kcal} = 367,000\,\mathrm{kgf\cdot m} \end{align}

But with so many units of energy was there any need to introduce one more? Yes, there was. The idea of energy is used in a great variety of fields of physics, so for the sake of convenience physicists introduced a new unit for each field. The same happened with other units of measurement. Finally, there appeared a need for a unified system of units (the SI system) for all fields of physics. Some time will have to pass, however, before the “old” units will make way for the favoured one, the joule. The kilowatt-hour is thus not thelast “outsider” that the reader will meet in his study of physics.

What are machines needed for? Obviously, to use sources of energy to do work: to lift loads, move other machines, or transport cargo or passengers. For any machine the amount of energy supplied to it and the output work done by it can be calculated. In all cases the work output is less than the work input: part of the energy is lost in the machine. The ratio of the work output for any machine to the work input is called efficiency and is usually expressed in percent. For example, a machine whose efficiency is 90 percent loses only 10 percent of the input energy. On the other hand, an efficiency of 10 percent means that the machine uses only 10 percent of the input energy.

The efficiency of a machine that transforms mechanical energy into work can be made very high if the unavoidable friction is reduced. We can bring the efficiency closer to 100 percent by improving lubrication, using better bearings, reducing the resistance of the medium in which the movement takes place, etc.

When mechanical energy is transformed into work, there is often an intermediate stage (as in hydroelectric plants), namely transmission of electric energy. Naturally, this stage introduces new losses. But these are small, so that even when this stage is present, the total loss in transforming mechanical energy into work can be brought down to a small percentage.